1. Field of the Invention
The invention relates in general to transport refrigeration systems, and more specifically to such systems having heating and cooling cycles which utilize hot compressor discharge gas.
2. Description of the Prior Art
Transport refrigeration systems for conditioning the loads of trucks and trailers have cooling, null and heating modes. The heating mode includes a heating cycle for controlling load temperature to a set point, as well as a heating cycle for defrosting the evaporator coil. When the system switches from a cooling or null mode into a heating cycle, hot compressor discharge gas is diverted by suitable valve means from a first or "cooling" refrigerant circuit which includes a condenser, a receiver, a heat exchanger, an expansion valve, an evaporator, and an accumulator, to a second or "heating" refrigerant circuit which includes the compressor, an evaporator defrost pan heater, the evaporator, the heat exchanger, and the accumulator.
To make more liquid refrigerant available during a heating cycle, a well-known prior art procedure pressurizes the receiver with the hot compressor discharge gas to force liquid refrigerant out of the receiver and into the refrigerant cooling circuit. A bleed port in the expansion valve allows this liquid refrigerant to flow into the evaporator during the heating cycle, to improve heating or defrosting capacity.
U.S. Pat. No. 4,748,818, which is assigned to the same assignee as the present application, improved upon this prior art receiver tank pressurizing procedure by eliminating the pressure line to the receiver, and by connecting the output of the receiver to the accumulator during a heating cycle. While this allowed some refrigerant to flow from the condenser to the receiver, a substantial amount of refrigerant was still being trapped in the condenser, especially at low ambients, e.g., below about 15 degrees F. (-9.44 degrees C.).
U.S. Pat. 4,912,933, which is assigned to the same assignee as the present application, improved upon the arrangement of the aforesaid U.S. Pat. No. 4,748,818. Similar to the '818 patent, the '933 patent connects the receiver and accumulator in direct fluid flow communication via a solenoid valve, but the connection is initially made prior to the initiation of a heating cycle instead of simultaneously therewith. After the flow path between the receiver and accumulator is established, the actual heating cycle is delayed for a predetermined period of time during which hot gas from the compressor continues to flow to the condenser. With the establishment of the direct fluid flow connection between the receiver and accumulator, and the low pressure at the accumulator compared with the normal pressure at the output of the receiver, the hot high pressure gas directed to the condenser during the delay period flushes out any liquid refrigerant trapped in the condenser, forcing it into the receiver and from the receiver to the accumulator. After the delay period, the heating cycle commences, with a supply of liquid refrigerant in the accumulator sufficient to provide near maximum heating capability during heating and defrost cycles, even at very low ambients. While this arrangement works well, during some operating conditions it has been found to return too much liquid refrigerant to the compressor, resulting in compressor slugging with resultant damage to the compressor which may lead to compressor failure. Providing a larger accumulator is an obvious solution to the slugging problem, but it adds to the cost, size and weight of the refrigeration unit.